1. Field of the Invention
The present invention relates to a pump apparatus and a hydraulic actuator.
2. Description of the Related Art
A hydraulic actuator used to, for example, change the inclination of an outboard motor with respect to a hull is provided with a selector valve in a channel between a pump and a cylinder apparatus internally partitioned into a lower chamber (first chamber) and an upper chamber (second chamber) by the piston; the selector valve directs a flow of a hydraulic fluid to the lower chamber or the upper chamber in a switchable manner. The selector valve includes an open valve located on a side of the selector valve which leads to the lower chamber and an open valve located on a side of the selector valve which leads to the upper chamber; the open valves are interlocked with each other. Each of the open valves is a combination of an actuation valve and a check valve which slide in a valve chamber.
The selector valve operates as follows. When a hydraulic fluid flows into a valve chamber for the open valve located on the side of the selector valve which leads to the lower chamber, the lower chamber-side check valve is opened under the pressure of the hydraulic fluid to cause the hydraulic fluid to flow to the lower chamber. In parallel with the operation of the check valve, the lower chamber-side actuation valve pressed by the inflow of the hydraulic fluid is displaced inside the valve chamber. The pressure of the displaced actuation valve displaces, via a communication path, the actuation valve in the open valve located on the side of the selector valve which leads to the upper chamber. Then, the displaced upper chamber-side actuation valve pushes and opens the upper chamber-side check valve to return the hydraulic fluid from the upper chamber to the pump. This operation extends the shortened cylinder apparatus to increase the inclination of the outboard motor.
On the other hand, when the hydraulic fluid flows into the valve chamber for the open valve located on a side of the selector valve which leads to the upper chamber, an operation opposite to the above-described operation is performed to feed the hydraulic fluid to the upper chamber, while returning the hydraulic fluid from the lower chamber to the pump. This operation shortens the extended cylinder apparatus to reduce the inclination of the outboard motor.
If the outboard motor has a weight larger than an expected value or air is mixed into a channel between the upper chamber in the cylinder apparatus and the check valve located on the side of the selector valve which leads to the upper chamber, when the tilted-up outboard motor is lowered, the outboard motor may move jerkily. This is because, as the cylinder apparatus shortens, the pressure in the above-described channel lowers excessively to prevent the upper chamber-side actuation valve from maintaining a displaced state, causing the check valve located on the side leading to the lower chamber to be repeatedly opened and closed.
Thus, to restrain this jerky motion, a narrowed orifice is provided in the channel (see, for example, Japanese Patent Application Laid-open No. H9-11987).
The above-described channel is formed in a manifold (housing) via which the pump and the cylinder apparatus are connected together, and thus, a machining operation needs to be performed on the manifold in order to form the narrowed orifice.
With the foregoing in view, it is an object of the present invention to provide a pump apparatus and a hydraulic actuator which allows jerky motion of the hydraulic actuator to be suppressed without the need to add a machining operation for providing a narrowed orifice.